WebOCalc - Free, easy power system help!

Hi everyone, I'm the author of WebOCalc, a Free and Open Source program that makes it very simple to choose power systems for almost any propeller-driven electric RC 'plane.

WebOCalc is not a motor calculator - instead, it helps you match the motor, propeller, gearbox ratio, and battery to the actual airframe. This ensures the best performance and lightest weight. On the other hand, motor calculators predict the performance of a specific motor, prop, gearbox, and battery, but ignore the airframe completely and tell you nothing about how well that combination will work on your particular airframe.

The philosophy behind WebOCalc is to keep the program very easy to use and as accurate as possible, without demanding lots of complex or unavailable data from the user. WebOCalc is also very flexible, it can help an absolute beginner to power system design get started, and it can also help experienced hobbyists use their knowledge to fine-tune a power system.

Edit: There was a bug affecting WebOCalc's ability to work directly off my website, it's now been fixed with the newest version, WebOCalc 0.9.8.4.

WebOCalc was first released in 2004. Since then I've put four years of development and improvements into the program. I've just uploaded the newest version, 0.9.8.4, which is the product of the last three months of work.

Previous versions of WebOCalc required some knowledge to get started: the user had to come up with suitable starting values for battery voltage and a couple of other parameters. The newest version includes "wizards" that are able to suggest good starting points for all these parameters, so a beginner can point and click his or her way to a successful power system.

Updates: here are some updates, and suggestions for improving WebOCalc that I received from readers in this thread:

There was a JavaScript bug that kept WebOCalc 0.9.8 from working unless you download it to your computer. The bug has been fixed and the fixed version (WebOCalc 0.9.8.4) is now on my website.

Sneasle points out that selecting "Slow 3D" can result in a warning about low pitch speed. I agree this can be confusing, and will attempt to fix it in a later version of WebOCalc.

Louis Fourdan points out that WebOCalc doesn't deal gracefully with finding a power system for the Easy Star. The fact is that the small prop diameter makes it impossible to match the power system and airframe in this case - all Easy Star's fly with mis-matched power systems - but I agree that WebOCalc should handle this situation more gracefully than it does now. I will attempt a solution in a later version.

NoFlyZone points out that the "suggest" llinks for pitch speed and thrust behave sufficiently differently from the "suggest" links for battery voltage, current draw, and motor Kv that they should probably be called something different. He also would like to be able to enter more significant digits into the textboxes ("wingspan", "wing chord", etc.)

Whitecrest suggests improving the visibility of the "here" links at the start of the voltage, current, and Kv help wizards.

Edit: version 0.9.9 released today, Oct 5th, 2008. Incorporates improvements based on Fourdan's, NoFlyZone's, and Whitecrest's suggestions above. I have to think a bit more about Sneasle's suggestion to find a way to implement it to my taste.
-Flieslikeabeagle

I've written a tutorial on using WebOCalc, complete with lots of screen shots. The tutorial is available on my website (same link as above, http://flbeagle.rchomepage.com, click Software, click WebOCalc, click WebOCalc Tutorial.)

The same tutorial is also included with every copy of WebOCalc - just click on the "Help" button once you have WebOCalc open in your browser.

For those new to this program it comes at matching props to planes in a different way to most programs and has now worked for me personally on planes ranging from 120W to 1055W.
Acrobatic planes, old time planes and 3D planes. It is an excellent addition to the problem of solving props-planes.
Just think about what prop you choose out of the options it offers.
It's a fantastic help. (And about time Flieslikeabeagle put it back out on public forums! )
Cheers
Glen

I've tried the commercial versions, and to tell you the truth, I run out of patience trying to fill in all the requested variables. They always seem to want to know some obscure variable like the price of tea in China, or the number of ergs in a dynamo.

I'll run this new version through it's paces and let you know what I find, Beagle...

Do you have a wing area or average wing chord for that model? It doesn't seem to be listed on BP Hobbies website, or in the pdf manual for the model. No matter, I'll guess at 10 inches unless you have better information.

Quote:

Originally Posted by sneasle

The suggested pitch speed is 22mph, which the calculator says is too low.
Suggested thrust is 25oz based on slow 3d 'flight mission'.

Suggest voltage, current, and Kv don't seem to work for me.

Hmm, that's odd. I put in your numbers and WebOCalc suggested props from 7" up to 12". Using 12" for first pass, WebOcalc suggested a 2S lipo pack and a Kv from 750 to 960 rpm/V. I put in 960 and WebOCalc immediately found a prop, and came back with the attached results. Gear ratio not yet 1.0, that's up to you to fine-tune if necessary (I'll show you how in the next few posts).

Attached you also see the warning message about the pitch speed: note that it doesn't say "too low", it says "...slightly lower than optimal. Consider a higher pitch speed for better performance." This is actually good advice! Using a bit too low a pitch speed for traditional "on the wing" flight is typical of slow 3D setups - you trade off pitch speed to maximize thrust available from a given amount of power.

However, I can certainly change the message if required...though I think in this instance it is saying the right thing? Perhaps what is needed is for the warning message to say something like "Okay for slow 3D, try faster speed for better traditional flight performance". But that gets long and a bit clunky!

Now to fine-tune the combo. Following WebOCalc's suggestion, I increased the pitch speed a bit by clicking on the "fast 3D" flight mission instead of "slow 3D". WebOCalc now suggested a 3S lipo pack at 14 A current draw (keep in mind this is a very inefficient motor, at around 50%, about as bad as a brushed motor when it comes to efficiency).

Clicking on the Kv wizard returned the answer to try a Kv between 860 rpm/V and 1100 rpm/V. This nicely spans the 1000 rpm/V of one of your suggested motors, so I put in 1000 rpm/V. The results are attached - we have a working combo. The gear ratio is not exactly 1.0:1, of course, that still needs fine tuning.

Looking at the suggested props, they are all low pitch/diameter ratios, meaning the pitch speed is still too low to achieve easily with a small outrunner (results would be better with a geared inrunner, as usual). So I raised the pitch speed a few mph more, to 35. Another click on "Calculate" returns a 10x7 prop at a gear ratio of 1.02.

Almost there...to get the gear ratio from 1.02 to 1.00, the motor needs a tad more torque...so I turned up the current from 14A to 15A, clicked on "Calculate" again, and voila! we have a working electric power combo to fly this model, arrived at in just a few seconds with a few mouse clicks.

I just used it last week to pick a prop for the newest plane I'm building, the ParkScaleModels Monocoupe 90A (sorta like the Charles Lindberg plane) at about 10 ounces AUW.

I found a little .9 ounce 1070 kV ourunner I had laying around with a constant current of 5 amps (8 amps burst). Being the conservative type, I figured I would ask the little motor to use no more than three amps at WOT on a 2s Li-Po.

Plugged the numbers into WebOCalc, told it I wanted 3 amps max, and it popped out the correct prop, with just the thrust and speed I was looking for, for nice scale flight.

I saw a photo of the ParkScaleModels Monocoupe in one of the RC rags recently. A beautiful model! And what a change from overweight plywood ARF's - only 10 oz RTF, that's amazing. A commerical ARF of the same size would probably weigh 30 oz or more. If you feel like posting a photo here, Chuck, I think it would be a great addition to the thread - let's have some pics of models whose powertrains were chosen with help from WebOCalc!

I realize ARF's have to survive much rougher handling and shipping and have to be built stronger in consequence, but it seems to me there is still usually far too much weight gain in the translation from kit to ARF. Probably because there is very little balsa in the ARF's I own - they're mostly plywood, except for a couple of ARF's made in the Czech Republic which actually are mostly balsa. Unfortunately, they're also pretty expensive.

Wasn't Lindberg's "Spirit of St. Louis" designed and built by Ryan Airlines, though, and not by Monocoupe?

Images

I saw a photo of the ParkScaleModels Monocoupe in one of the RC rags recently. A beautiful model! And what a change from overweight plywood ARF's - only 10 oz RTF, that's amazing.

<snip>

Wasn't Lindberg's "Spirit of St. Louis" designed and built by Ryan Airlines, though, and not by Monocoupe?

Hi Beagle,

Actually, it's going to come in closer to 9 ounces than 10, and I picked a current rating of 4 amps for the motor, not three. I was reciting the figures earlier from work, and now I have them in front of me. A 36 1/8" wing span and 190 in² should give me a wing loading of about 6.8 ounces per ft².

Actually, I know nothing of Lindberg's plane, and was under the assumption that it was akin to the MonoCoupe 90A LOL. All I know is when I saw it, I had to have it!

Mine is still under construction, but here's a pic of what I 'hope' it looks like when I'm done. (crossing fingers)...

WebOCalc gave me an APC 8x6E prop, which will be perfect. About 8.2 ounces of thrust if I recall correctly.